This invention relates to selective detection of specific compounds and in particular to a simple method of calibrating or checking operation of a vapor detector such as an explosives vapor detection system.
Detection of explosives carried by persons or concealed in buildings, airplanes, cars or other locations can be vital to prevention of injuries and damage to property. However, detection by direct searching is quite costly and time-consuming, can at times be dangerous, and can also be susceptible to error. Thus, it is desirable to detect explosives somewhat indirectly, as by their presence in very small amounts of vapors in air or other gases which have been in contact with explosives in solid or liquid form.
To be effective, devices for detecting vapors of selected compounds such as explosives in air must fulfill several requirements. They must, of course, be reliable. Also, they need to be highly sensitive in order to detect the minute quantities (parts per quadrillion (10.sup.15)or less) present in vapors and which in turn may indicate the presence of much larger quantities of the compounds. It is essential that explosives detection systems be very selective so as to prevent or minimize false alarms which would result from detection of compounds which are not explosives, and yet be highly reliable so that no explosives present are overlooked or not detected. In certain applications, such as screening persons for possession of explosives, detectors must operate rapidly--they must determine, essentially in real time, whether explosives are present--and they should also be as non-intrusive as possible. For many situations, it is important that the detector identify the specific explosive detected. Other characteristics which may be important in an explosives detector are that it be portable, rugged, and able to function in harsh environments.
Various systems are known for detecting specific compounds such as explosives, but none have provided the combination of selectivity, sensitivity, reliability, and rapid response needed for an effective and reliable detector. Systems such as electron capture detectors, mass spectrometers, ion mobility spectrometers, and nitric oxide chemiluminescence analyzers have been employed for detecting explosives, as have certain animals (notably dogs). The systems may perform satisfactorily if provided with high or moderate levels of certain explosives vapors and if allowed ample time for analysis. However, they generally are slow and also fail to provide the selectivity to distinguish explosives from various other compounds, particularly nitrogen-containing compounds, whose vapors may be present along with the explosives. The selectivity of such systems decreases as the concentration of explosives decreases and is a significant drawback in detection of low levels of explosives. As a result, non-explosives such as halogenated solvents, nitorsamines, perfumes, nitrogen oxides (NO.sub.x), and phthalates interfere with, and may give false readings instead of, accurate detection of explosives.
It is an object of the invention to provide a simple method of calibrating a vapor detector.
It is an object of the invention to provide a method of calibrating a vapor detector wherein vapors of specific compounds may readily be generated in remote locations.
It is an object of the invention to provide a method of rapidly calibrating or checking response of a vapor detector which utilizes pre-packaged, sealed samples of treated sorbent material as a source of vapors and avoids the need to inject liquids into a gas stream.
It is an object of the invention to provide a method of calibrating or checking response of an explosives vapor detector which utilizes a vapor collection technique similar to that employed by the vapor detector.